1. Field of the Invention
The present invention relates to a degaussing circuit for a CRT (cathode ray tube) used in a color television, a color monitoring device, a color display device, or the like.
2. Description of the Prior Art
Generally, although CRTs have shadow masks, there will be still some color shading observed due to an undesirable magnetic field, such as the earth's magnetic field, which affects the CRT. In order to eliminate the unwanted magnetic field, a degaussing circuit has been proposed which includes a degaussing coil with a positive temperature coefficient thermistor inserted with respect to the power source. The degaussing coil is usually provided by one loop wound on the back surface of the CRT, or by two loops provided on the upper half and lower half of the back surface of the large size CRT to increase the degaussing effect.
Generally, the positive temperature coefficient thermistor made of a material containing barium titanate has some capacitive component C, which is about 4-40 nF when measured during the high impedance stable condition obtained after the completion of the degaussing operation as effected immediately after the commencement of the power supply. Since the degaussing coil itself has an inductance of about 10-20 mH, the degaussing circuit defined by a combination of the positive temperature coefficient thermistor and the degaussing coil will have a resonance frequency (fo=1/2.sqroot.LC) which sometimes takes a value of about 15-16 kHz. Since the horizontal scanning frequency of the deflecting yoke provided around the electron gun of the CRT is 15.75 kHz, the signal generated by the deflecting yoke may resonate with the signal through the degaussing coil such that the magnetic flux produced by the deflecting yoke links with the degaussing coil to generate induction current, which may undesirably rise to about 100 mA (normally several mA), in the degaussing coil, resulting in undesirable picture fluctuation even after the completion of the degaussing.
Although it is possible to prevent the degaussing coil from being adversely affected by the magnetic field produced by the deflecting yoke by providing a suitable shield, such a shield results in high manufacturing cost.